The present invention relates to laser-equipped machine tools, and more particularly to a heavy-duty laser plate cutting machine.
In the following paragraphs, background information, and information summarizing the invention will be presented together so as to convey a coherent view of the significance of the invention.
Laser-equipped machine tools are often used to cut parts from sheet metal and plate. In such machine tools a laser beam, concentrated by a focusing lens or mirror to a small diameter spot, or focus. The laser beam is directed to position the focal point above, at or below the surface of the material to be cut. The laser beam is directed by the focusing optic through a nozzle disposed immediately above the workpiece, with a pressurized gas being directed through the nozzle, typically coaxially with the laser beam, to assist the cutting process. The pressurized gas interacts with the laser beam and material, facilitating the cutting process, and creates a high velocity stream that carries the melted material away from the cut.
Laser-equipped machine tools are usually Computer Numerically Controlled, and are manufactured in many configurations and sizes and with lasers of various types and power. The present invention relates to heavy-duty plate lasers, such as those which are capable of cutting steel plate on the order of one-inch thick or more at production cutting rates on the order of 24 inches per minute. The present invention is directed to a machine having those capabilities and, in some instances, sufficient adaptability to also efficiently handle lighter materials, such as sheet metal. In the most preferred embodiment, a xe2x80x9cflying opticxe2x80x9d configuration is utilized. In that configuration the cutting head is adapted for movement along one axis, such as the Y-axis which is mounted on a bridge adapted for movement along an orthogonal, X-axis. The work is supported on a stationary pallet or table below the bridge. Movement of the cutting head is coordinated with movement of the bridge to define a precise path on the part. The cutting head and laser are controlled to pierce and cut thereby forming holes and shapes in the material, and then to cut the part from the material.
In configuring a versatile machine tool capable of cutting heavy plate, it is highly advantageous to provide for the use of focusing optics with different focal lengths. An optic with one focal length can be used for cutting thick plate, and another with a different focal length can be used for cutting thinner materials. The ability to change focal lengths is an important feature in a heavy-duty plate machine adapted to also cut lighter materials.
The focal length of the optic contributes to the diameter of the focal spot and thus the energy density, Watts per unit area, at the focal spot. Shorter focal length optics create smaller focal spots having higher energy densities. The focal length of the optic also contributes to depth of focus of the focal spot with longer focal lengths having greater depth of focus. Shorter focal length optics are advantageous for cutting thinner materials while longer focal length optics are advantageous for cutting thicker material. The focal length of the optic and the power level of the laser contribute to the energy density remaining in the laser beam at distances beyond the workpiece during various stages of the cutting process.
Adapting high power lasers to cut thicker materials leads to using focusing lenses with longer focal lengths. Below the focal point, a laser beam expands at approximately the same rate that it was focused. For example, if a 35 mm diameter laser beam is focused by a lens with a 10xe2x80x3 focal length, then, 10xe2x80x3 below the focal point, unless absorbed by the material cut, the beam would be approximately 35 mm in diameter again. Twenty inches below the focal point the beam would be roughly 70 mm in diameter. This remnant diverging beam from high power lasers has considerable capability to cause damage. For example in certain tests, a 0.125xe2x80x3 thick aluminum plate was scuffed with steel slag, then a 38 mm diameter 5500 Watt beam was directed at the surface. The aluminum was cut through in 40 seconds. Similar tests were made with 0.25xe2x80x3 inch thick stainless steel and carbon steel. Both were cut through in well under a minute. These tests indicated that a scrap collection bed underlying the cutting area of a high power laser system, with long focal length optics in use, would be at considerable risk of being damaged by the remnant laser beam.
In a laser cutting machine, the laser beam is produced in a laser generator and is directed along a beam path via a beam delivery system. A beam delivery system is a collection of optical elements, such as reflective mirrors and transmissive optics, which may redirect the beam, alter the propagation characteristics of the beam or focus the beam. The beam delivery system is enclosed for safety and for control of the beam path environment within. The laser beam is concentrated by a focusing lens or mirror to a small diameter spot, which is directed to an appropriate position relative to the surface of the material to be processed.
In most implementations, the laser beam exits the laser through an output coupler, a partially transmissive and partially reflective optical element which seals the laser cavity and transmits a portion of the beam out of the laser cavity or resonator. The beam is then directed along a beam path to a focusing optic in a processing head near the work. In most cutting applications, the beam is directed by the focusing optic through a nozzle disposed immediately above the workpiece to be cut. A pressurized gas is also directed through the nozzle, typically coaxial to the beam, to assist the cutting process. The pressurized gas serves to facilitate and/or shield the cutting process, and creates a gas stream which helps remove vaporized and molten material from the cut or kerf. Kerf refers to the zone of material which is acted upon and removed by a cutting process. Kerf width refers to the width of the slot created by the cutting process, such as the width of the slot cut by a laser beam as it moves along a path.
Key factors in laser processing include the diameter of the focus spot and the position of the focus spot relative to the material to be processed. The control of these focal characteristics is critical to maintaining the quality of the process. During processing, unintended deviation in the focus spot size and position may produce a deterioration in process quality and may even cause the process to fail.
The first of two main factors which influence the focus characteristics is the diameter of the laser beam at the focal optic. Due to diffraction, the minimum focal spot diameter, for a given focal length optic, is limited. Diffraction causes light beams to diverge or spread transversely as they propagate. As the input laser beam diameter, of a typically diverging beam, increases at a given focal optic, the focus spot diameter decreases due to a decrease in diffraction. In addition, as said input laser beam diameter increases for a given focal optic, the focus spot position shifts closer to the focus optic.
The raw laser beam, issuing from the laser resonator, exhibits the characteristic of divergence. The beam diameter will change as a function of the distance from the output coupler. Typically, as the processing head moves over the processing area the distance from the output coupler to the focal optic will change. When a large processing area is required, some method of maintaining the proper beam diameter must be employed in order to avoid significant changes in focus diameter and position.
Additionally, changes in the output power level of the laser will affect the divergence of the output beam. The largest effect on beam divergence comes from the thermal loading of the output coupler which produces thermal lensing. Thermal lensing is distortion of an optical component caused by heat absorbed from the input beam. The absorbed portion of the beam causes expansion of the output coupler such that the curvature of the surface changes. The expansion causes a change in the divergence of the output beam thereby changing the beam size at any given distance from the output coupler. The rate and amount of distortion is dependent upon the power of the beam, optic contamination, thermal conductivity of the optic and its cooling system and the length of time the beam is applied. Upon reaching thermal equilibrium, when absorbed heat is in balance with that removed by the lens cooling system, the shape of the optic surface remains constant. When the beam is turned off, the optic surface gradually relaxes and returns to its original shape. When a high output power laser is required, some method of maintaining the proper beam diameter, in a time dependent response to output power changes, must be employed if significant changes in focus diameter and position are to be avoided.
The second of two main factors which influence the focus characteristics is the distortion of the focus optic due to heat absorption. In a manner similar to that described for the laser output coupler, thermal lensing occurs in the focus optic. The expansion of the focus optic changes the effective radius of curvature which causes the focal spot to shift relative to the focus optic. When a high output power laser is required, some method of maintaining the proper focal position, in a time dependent response to input laser power changes, must be employed if significant changes in focus position are to be avoided.
Proper focal position is very important in cutting heavy plate. In initiation of a cut, the plate must be pierced, and a preferable piercing technique requires xe2x80x9cdrivingxe2x80x9d the beam through the plate. This can be accomplished by altering the position of the focal spot, by actually moving it into the plate as the piercing operation progresses. Furthermore, in cutting different types of materials, it is often useful to alter the focal spot position with respect to the surface of relatively thick materials so as to optimize the quality of the cut.
Turning now to the divergence issue mentioned above, one method employed to reduce the divergence of the laser beam is to expand or magnify it with a collimator. The rate of divergence of a beam is reduced in inverse proportion to the amount it is magnified. If a beam is magnified by 125 percent its rate of divergence is reduced 20 percent. If it is magnified by 200 percent its rate of divergence is reduced by 50 percent.
Collimators are optical devices, also known as beam expanders and condensers. Such devices also have other characteristics and functions known to those skilled in the art. Manufacturers of laser optics publish literature providing information on design variations and examples of use. One example of such literature is the II-IV Incorporated publication, Beam Expander-Condensers, published 3/92. Collimators can be constructed of transmissive optics such that the beam is passed through the optics. Such collimators are commonly used in laser-equipped machines up to about three kilowatt power levels and sometimes above.
Collimators used on low powered lasers are designed or adjusted to magnify the beam a given amount, and then locked in place. Use of transmissive collimators with lasers having power levels above three kilowatts becomes increasingly problematic due to thermal lensing and due to limits on the energy density that transmissive optic materials can withstand. Impurities within optical materials, crystal growth conditions, surface contamination and surface imperfections are primary causes for a portion of a laser beam to be absorbed and converted to heat within a transmissive optical element.
The distortion produced by thermal lensing can influence the divergence and mode quality of the beam passing through or reflecting off of the optical delivery and focusing components and thereby cause detrimental shifts of focus position. Thermal lensing is a greater problem with transmissive optics. For example, when a high power beam is directed at the curved surface of a plano-convex focal lens, which has a curved first surface and a flat second surface, the absorbed portion of the beam causes expansion of the lens such that the curvature of the surface changes. The expansion reduces the effective radius of curvature which causes the focal spot to shift back from the material or closer to the lens. The rate of curvature change is greater toward the center of the lens due to the power distribution of the incident laser beam. Therefore, the heating and the expansion is greater toward the center of the lens. Fixed collimators constructed of transmissive optics are very susceptible to thermal lensing which reduces their effectiveness for use with high power lasers.
Collimators are also constructed of reflective optics, combinations of flat and shaped mirrors, such that the light beam is reflected from the optical elements. Reflective optical elements are typically manufactured from materials, such as copper, which can withstand greater energy densities without damage. Also, when compared to transmissive optics, thermal lensing of reflective optics is not as severe. Thus reflective collimators are more suitably used in high power laser applications. However, a fixed, reflective collimator cannot compensate for the thermal lensing of a laser output coupler nor for the thermal lensing of a focal optic.
In view of the foregoing, it is a general aim of the present invention to provide a high power laser-equipped machine tool having the capability to cut relatively thick plate at production rates.
It is a more detailed object to provide a high power laser-equipped cutting machine of the foregoing type which is sufficiently versatile to be able to also cut sheet metal at production rates.
An objective of the present invention is to provide a laser-equipped cutting machine having a combination of features and functions adapted to cutting a relatively thick plate at production rates, with sufficient versatility to function on thinner materials at commercially acceptable cutting rates.
It is a feature of the preferred embodiment of the invention that the machine tool utilizes a high power laser and a flying optic beam delivery system for delivering a high power beam to a workpiece while still providing relatively fast feed rates. The beam delivery system includes automatic beam compensation to maintain precise control of the focus spot size and focus position of the laser beam while accommodating changes and variations in the optical system due to the energy in the beam and distance between the laser generator and the cutting head. The focusing lens carrier system in the cutting head provides means for ready adjustment of the position of the focal spot relative to the work to handle focal spot position adjustments needed for cutting thick plate. Positioned below the workpiece is a slag collection bed, configured so as to be able to occasionally absorb the power of the laser beam without permanent damage.
Other objects and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.